This invention relates to a directional loudspeaker unit, and more particularly to a loudspeaker unit having a horn with an acoustic output having a substantially phase-coherent radiating wavefront.
Adding a horn to a loudspeaker increases acoustic output of an associated driver unit in non-uniform manner, by causing the maxima (greatest value) of acoustic output to occur typically in the lower octaves of the operating band (the operating band is also referred to as the “passband”). The position of the maxima with respect to frequency is determined by the geometry of the horn, primarily the mouth area and depth of the horn. As the frequency increases from the frequency associated with the maxima in acoustic output, the output of a loudspeaker with a horn tends toward that of a loudspeaker without a horn. Transition smoothness of acoustic output as frequency increases from the frequency associated with the maxima depends on horn geometry, and primarily on the contour of the horn walls.
The shape of the horn walls, mouth area and horn depth also determine how acoustic output from the horn radiates into free space. If the acoustic radiation into free space up to a specific angle from the central axis of the horn in a particular (e.g. horizontal) plane is consistent over a frequency range of the horn, the horn is said to have a “constant directivity” in that plane. A known way of obtaining more nearly constant directivity in the upper frequencies of the passband is to have a width of the horn narrow from a throat width to a particular dimension related to the wavelength of the upper frequencies. Here, the term “throat” refers to the junction between the driver and the horn mounted on the driver. The plane perpendicular to the horn axis at the narrowed width of the horn is termed the “diffraction slot”.